1. Field of the Invention
The present invention relates to an optical source for a communications system and finds particular application in supplying high frequency signals to a photodetector.
2. Related Art
It is known to use a semiconductor laser structure as a signal source in optical communications. Such structures usually comprise an active layer of semiconductor material into which electrical carriers are injected by applying a drive current. The active layer is provided with optical confinement and feedback. The injected carriers produce photons in the active layer which oscillate and multiply by laser action to produce an optical output, often from an end facet of the laser structure.
It is known that, in distributed feedback (DFB) lasers, it is possible to superimpose modulation on the optical output of a laser by varying the drive current. Because of physical limitations of the devices, this has only been done at rates up to about 20 GHz. It is also known that as the drive current power increases, optical sidebands are seen, separated by the optical modulation frequency. As the optical frequency deviation of the source increases, more sidebands are produced, reflecting an overall spectral spreading of the modulated source.
On a photodetector a received electrical signal is produced which is given by the (phasor) sum of the beat signals generated between all pairs of optical sidebands. The received electrical signal therefore contains components at multiple harmonics of the modulating frequency. However, a frequency modulated (FM) or phase modulated (PM) optical signal results in an electrical signal containing only a D.C. (zero frequency) component: the summation of beat signals between sidebands exactly cancels to give zero for the electrical signal components at the modulation frequency and higher harmonics thereof. Hence, although high frequency beat signals are present, they do not manifest themselves at a conventional photodetector based receiver. In the paper entitled "35 GHz Microwave Signal Generation with an Injection-locked Laser Diode", published in Electronics Letters 29th Aug. 1985 vol 21 no. 18, L Goldberg et al describe a system for generating 35 GHz signals by injection locking a slave laser to sidebands of a modulated master laser output. Using a slave laser which has free-running modes separated by a frequency difference which equals a selected beat frequency between sidebands of the master laser, an output is generated from the slave laser which comprises two relatively high amplitude signals separated by that same beat frequency. At a receiver, the beat frequency appears. However, two laser devices are required and it can be difficult to achieve a workable arrangement, it being necessary to provide close wavelength control. It is also difficult to achieve requisite modes in the slave laser, spacing of modes being limited by the length of the device.